1. Origin of the Invention
This invention is the result of the co-inventors' recognition of a long-standing, unresolved problem and the realization of means for solving that problem. The invention is assigned by the co-inventors to the co-inventors' corporate employer.
2. Field of the Invention
The invention is related to the field of digital data transmission, and, more particularly, to digital data transmission in a T-1 carrier system, wherein a user may telephone over a telephone port or transmit data over a data port. Such signals from a user location are respectively transmitted in analog or digital format to a channel bank, which is commonly associated with telephone operating companies' digital loop T-1 carrier systems. Typical of such channel banks are types known as D1, D4 and SLC banks.
3. Description of the Prior Art
In the evolution of plain, ordinary telephone systems to a shared use of telephone and data, the telephone companies have responded by numerous equipment changes. First, the telephone companies reduced the number of pairs of wires by the introduction of carrier systems. For example, with a group of 24 plain, ordinary telephones, the number of dedicated copper pairs would also be 24 (i.e., one pair for each telephone). The number of required pairs of wires were reduced when the telephone companies introduced the carrier system approach. In the T-1 approach, 24 pairs of copper wires were reduced to a single "T-1" carrier span, wherein each T-1 span is a conditioned two-wire transmit line and a conditioned two-wire receive line. The T-1 span transmits digital signals at a rate of 1.544 Mb/sec., and channel bank equipment is provided at each end of every T-1 span. Each channel bank includes analog-to-digital and digital-to-analog converters for adapting the signals from ordinary telephones to central office digital equipment and back to telephones.
With the advent of the more recent extended digital central offices, the number of analog-to-digital conversions were reduced. Digital T-1 spans were extended from a digital central office to a remote terminal location. That remote terminal location may, for example, be assigned to several large office buildings having a high number of telephones. The number of T-1 spans would be chosen in accordance with the telephone density. At the remote terminal, an analog-to-digital conversion is still required in order to provide an interface from plain, ordinary telephones to digital system equipment.
In today's extended digital systems a user requires one channel unit port for telephone and a different channel unit port for data. The data port in this prior art system is a dedicated data port in that the central office can send data only to a predetermined location. The central office was thus able to assure that no analog portion of a dial-up network would interfere with the data transmission. Neither voice nor dial-up capability was available over that dedicated data port.
In the use of T-1 spans, channel bank units are at a central office terminal or at a remote terminal. Time slots are assigned for each input port, and each input port is associated with an analog telephone. A data terminal at a user's location can also have a time slot assigned to it, in which the data terminal time slot is on the above-noted available predetermined basis. An assigned time slot, for ease and simplicity of explanation, is referred to in this application as a telephone "line".
The remote and central office terminals employ signalling bits, which are transmitted over the T-1 spans for control purposes. When one port is used for voice, it is known to rob the least significant bit from every sixth sample for signalling purposes. That robbed bit is forced to a given binary condition to indicate a signalling function such as on-hook/off-hook, or ringing of the telephone.
If no voice is being transmitted, a high-speed data port could be connected between predetermined users over a T-1 span. The robbed bit technique is still employed for control functions by the telephone company. That robbed bit requirement reduces the data port for a T-1 span to its maximum rate of 56 kb/sec.
Digital telephones and a digital private branch exchange (PBX) are today well known. Various semiconductor manufacturers supply off-the-shelf chip sets to build digital telephones and PBX line cards. A Motorola Inc. manual entitled "Telecommunications Device Data", copyrighted in 1984, at pages 2-279 through 2-332 and 3-23 through 3-36, is a typical description of such chip sets.
The digital PBX, as described in the Motorola manual and as commonly understood, keeps data separate from digitized voice. The Motorola PBX chip also has a mode to add voice plus data by robbing the eighth bit, but it is for communication within the local PBX premises. The PBX chip routes data locally among the PBX users, but does not directly pass that data out over any terminal or T-1 spans into the telephone system. If a PBX user desires that data be transmitted over a T-1 span, either a modem for an analog telephone time slot, or a time slot assigned for data transmission to a predetermined location, must be employed. Simultaneous voice and data over one telephone "line" or time slot over T-1 spans is not available in such PBX systems.
The telephone companies are, today, hopeful of providing an all-digital network with universal standards. Such hopes have been slow to materialize, and most of the voice and data industry recognize that analog system links are still prevalent in routing of any dialed-up telephone system.
A desperate need has thus gone unfulfilled in that digital telephone user requirements for simultaneous dial-up voice and data over T-1 spans have not been satisfied. This invention meets most of today's data requirements, plus supplying simultaneous or alternate voice over a dial-up call placed over T-1 spans in a highly efficient manner.